Sometimes post imports in WordPress can create duplicate custom fields for posts that already exist. This will delete custom fields on posts the have the same key and value. A post type restriction is included as an example of further narrowing.

delete-dup-custom-fields.sql

DELETE o
/* SELECT p.post_title, o.*, pmi.* */
FROM wp_postmeta AS o
INNER JOIN wp_posts AS p ON p.ID = o.post_id 
INNER JOIN wp_postmeta AS pmi ON pmi.post_id = o.post_id AND pmi.meta_key = o.meta_key AND pmi.meta_value = o.meta_value
WHERE p.post_type = 'post'
AND pmi.meta_id < o.meta_id